Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Therein, a subset of all the engine cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. An engine control system may disable a selected group of cylinders, such as a given bank of cylinders that have cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, and/or through the control of a plurality of selectively deactivatable fuel injectors that affect fueling of the cylinders of the given bank. The other bank of cylinders may not have cylinder valve deactivation capabilities and may continue to fire. Further improvements in fuel economy can be achieved in engines where all the cylinders have deactivation mechanisms so that the specific cylinders that are deactivated can be changed every cycle. For example, the effective displacement of the engine may be varied by skipping the delivery of air and fuel to certain cylinders in an indexed or rolling cylinder firing pattern, also referred to as a “skip-fire” pattern. One example of a skip-fire engine (also referred to as a rolling variable displacement engine (VDE) system) is shown by Tripathi et al. in U.S. Pat. No. 8,651,091. Therein, an engine controller may continuously rotate which particular cylinders receive air and fuel, which cylinders are skipped, and how many cylinders events the pattern is continued for. By skipping air and fuel delivery to selected cylinders, the active cylinders can be operated near their optimum efficiency, increasing the overall operating efficiency of the engine. By varying the identity and number of cylinders skipped, a large range of engine displacement options may be possible.
During cylinder operation, typical engine control methods use volumetric efficiency characterization to estimate the cylinder air charge. This estimation is required prior to every cylinder induction event to determine the amount of fuel to be injected in the corresponding cylinder, as well as to estimate the torque contribution from that cylinder. Volumetric efficiency information may also be used to compute engine air flow and coordinate the operation of air-path actuators, such as an intake throttle. Further, some control methods may use volumetric efficiency information to compute estimated intake manifold pressure from engine air flow, throttle air flow or mass air flow sensor values.
In conventional VDE systems, except during the transition from deactivated to activated, the active cylinders are always inducting and their volumetric efficiency characteristics are similar to the case when all cylinders are inducting. In comparison, for engine systems having rolling VDE capabilities, cylinder aircharge estimation may be difficult due to the numerous firing patterns that are possible. One example approach for estimating the air charge of a cylinder in an engine configured with rolling cylinder firing is shown by Jankovic in US20160146139. Therein, an air charge estimate for a cylinder is adjusted based on whether the subsequent cylinder in the current engine cycle was fired or skipped in the preceding engine cycle. Thus, this approach accounts for the interaction of a cylinder with other cylinders.
However, the inventors herein have identified a potential limitation with such an approach. The above mentioned approach is applicable to conditions where there is valve overlap at the current cylinder thereby allowing other cylinders to influence the residual gas during valve overlap by affecting the instantaneous intake and manifold exhaust pressures. This is applicable to conventional VDE systems where the exhaust is deactivated first followed by the deactivation of the intake valve, resulting in trapping of high pressure exhaust gas. In rolling VDE systems, during cylinder deactivation, an intake valve is deactivated first followed by the deactivation of an exhaust valve of the selected cylinder. Therefore the cylinder completes the exhaust stroke from the prior cycle before the valves are deactivated and the cylinder is isolated. This results in trapping of exhaust gas at a pressure closer to the exhaust manifold pressure. When the cylinder is reactivated, the intake valve opens first, during an intake stroke, and the exhaust valve opens later, near the end of an expansion stroke. This order of deactivation and reactivation of valves results in a significant change in the interactions between the intake manifold and the exhaust manifold resulting in a cylinder-to-cylinder air charge variation. In one example, there may be up to +/−12% variability in cylinder-to-cylinder air flow. If the airflow is not correctly estimated, cylinder-to-cylinder air fuel ratio control may be impacted and there may be significant air fuel ratio imbalance between cylinders. In addition, errors may be introduced in throttle control, residual gas estimation, and torque estimation. The effect may be exacerbated due to the frequent switching between active and deactivated cylinders. As a result, engine operation and exhaust emissions may be adversely affected.
In one example, some of the above issues may be addressed by a method for an engine comprising: adjusting an air charge estimate for a cylinder on a current engine cycle based on an induction pattern history of the cylinder, including an induction state of the cylinder on an immediately previous engine cycle; and injecting fuel to the cylinder in response to the adjusted air charge estimate. In this way, by adjusting an air charge estimate based on a firing history of a cylinder, air charge may be estimated more reliably even as cylinder firing patterns vary.
As one example, each cylinder of an engine may be configured with a selective deactivation mechanism. Based on engine speed and load, one or more of the cylinders may be selectively deactivated, and further, the number and identity of cylinders deactivated on each engine cycle may be varied. For example, an induction ratio that efficiently meets the torque demand may be determined and then a cylinder deactivation pattern may be selected based on the induction ratio wherein active cylinders are evenly spaced for acceptable NVH characteristics. Prior to a cylinder induction event, a volumetric efficiency of the cylinder (to be fired) may be calculated using one of two aircharge calibrations, the aircharge calibration selected based on the most recent induction state of the cylinder. For example, it may be determined if the cylinder to be fired was fired or skipped in the last engine cycle. If the cylinder was skipped, it may be further determined as to how many cycles the cylinder was skipped for (that is, when the last induction event occurred in the same cylinder). If the cylinder was fired in the last cycle, on the current cycle, aircharge may be estimated using a first aircharge characterization. Else, if the cylinder was skipped in the last cycle, on the current cycle, aircharge may be estimated using a second, different aircharge characterization. In the case where the cylinder fired in the previous cycle and is firing in the current cycle, the presence of two consecutive fires results in the intake and exhaust valves being open at the same time, resulting in exhaust residuals reaching the intake runner in addition to occupying the clearance volume (or cylinder volume at the exhaust valve closing). Subsequently during the intake stroke these exhaust residuals are inducted back into the combustion chamber thereby reducing the fresh air trapped in the cylinder. The first aircharge characterization may compensate for this effect. In the case where the cylinder was skipped in the previous cycle and is firing in the current cycle, due to the order of valve deactivation relative to valve reactivation, the intake and exhaust valves are not open at the same time. As a result, there may be no blow-back of exhaust residuals into the intake runner. Only the clearance volume (or the volume of the combustion chamber when exhaust valve closes) is occupied by the residual gas. Thus, a larger amount of air is inducted during the intake stroke as a result of lower amount of residual gas. The second aircharge characterization may compensate for this effect. In some examples, an instantaneous estimate of intake manifold pressure at a time of intake valve closing (IVC) may be used to further enhance the first and second aircharge characterizations. Following the determination of an amount of air entering the cylinder using the selected aircharge characterization, fuel may be accordingly injected into the cylinder.
In this way, aircharge estimation for an engine configured with rolling VDE capabilities is improved. By compensating for the presence or absence of valve overlap based on the induction history of a cylinder in previous cycles, cylinder volumetric efficiency may be calibrated more accurately and with less computation intensity. By using the induction history of a cylinder to update the aircharge estimate for the given cylinder, differences in the exhaust residuals retained in the cylinder due to the differences in intake and exhaust valve operation during cylinder deactivation and reactivation events can be better accounted for. In addition, an intake manifold pressure difference (that drives intake air into the cylinder) arising from the different order of valve operation during cylinder deactivation and reactivation events can be used to more accurately determine the amount of air trapped in a cylinder at BDC. By reducing cylinder-to-cylinder air-fuel imbalances, engine performance and exhaust emissions may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.